The invention relates to the field of fiber optics, and more particularly to birefringence in single-mode fibers.
Linear birefringence in single-mode optical fibers arises from either geometrical effects or anisotropic stresses in the core. In extreme cases, the core may be intentionally distorted into an ellipse or subjected to strong internal anisotropic stress and result in polarization maintaining fiber. All fiber has some core ellipticity, though in modern single-mode communications fiber it is limited to less than 1%. There is a great desire to limit core ellipticity to minimize the linear birefringence that causes polarization mode dispersion (PMD). This effect limits the maximum information bandwidth that can be transmitted on an optical link of a given length due to pulse broadening. Another cause of linear birefringence is stress caused by bending of the fiber, although this can be minimized by avoiding small radius bends.
When a single mode optical fiber is used for current sensing by forming a loop around an electrical conductor, linear birefringence can be shown to cause a reduction in the sensitivity from that which is anticipated due to the Verdet constant of the fiber. The sensitivity, S, can be shown to have the following approximate form:
S=xcfx81sin xcex4/xcex4, 
wherein xcfx81=VNI:
V=Verdet Constant in rad/Amp;
N=number of turns in the fiber coil;
I=current in conductor enclosed by coil; and
xcex4=xcex2NDxcfx80, 
xcex2=linear birefringence in rad/m, and
D=diameter of the fiber coil in meters.
Since NDxcfx80 is the length of the fiber in the coil (neglecting pitch of the coil turns), the sensitivity decreases along the length of the coil. Since the linear birefringence due to the various stresses is a function of temperature in a relatively unpredictable way, the sensitivity (scale factor) of the coil is difficult to control, thereby also making calibration difficult.
It has been shown that the introduction of circular birefringence in a single-mode fiber Faraday-effect magnetic sensing coil can mitigate the effects of incidental linear birefringence which are due to such phenomena as bending stress, residual ellipticity, and compression due to the coating of the fiber. If the amount of circular birefringence is substantially greater than the linear birefringence per unit length, then the sensitivity of the coil per turn becomes essentially independent of the number of turns, and also substantially independent of temperature. This permits the construction of accurate sensing coils whose sensitivity is substantially independent of temperature. This method differs from the method of annealing the fiber coil to relieve the mechanical stresses. (Though the Verdet constant may also vary with temperature and affect sensitivity, this effect may be dealt with separately.)
Circular birefringence can be introduced in different ways. A fiber can be twisted (thereby putting it in torsion) or the preform can be spun when the fiber is made so that the twist is xe2x80x9cfrozenxe2x80x9d in the fiber. Both of these effects can be shown to reduce the effect of linear birefringence on PMD and on the current sensitivity.
It has been shown that torsional-stress-induced circular birefringence introduced by twisting the fiber can be used to xe2x80x9cswampxe2x80x9d the linear birefringence. The fiber can then be annealed to eliminate or at least reduce any additional stress-induced linear birefringence. In another example disclosed in U.S. Pat. No. 6,023,331 to Blake, which is incorporated herein by reference, a helical coil of fiber was wrapped around a torus. The fiber is subjected to a number of twists to establish circular birefringence which tends to cancel out the effects of linear birefringence. A twist in this context may be an essentially constant rotation per unit length of the fiber, along the length of the sensing coil. Blake suggested that specific values of the circular birefringence exist for closed-loop fiber-optic sensors, where the linear birefringence can be cancelled exactly, producing a zero scale factor error. These specific values of the circular birefringence depend on the pitch angle "THgr") and the radius of the helix R. For a coil with N turns, the zero scale factor error solutions may be achieved by:
T=N*sin "THgr"*cos "THgr"/R, 
where T is the rotation caused by circular birefringence.
Using the above relations allows the fabrication of Faraday-effect sensing coils that do not require annealing. However, achieving this effect requires a specific number of turns or twists (determined by the equation for T) in the helix which is wound around the torus mandrel. Blake also notes that introducing a large number of twists in the optical fiber is impractical as such a tightly-wound fiber will creep over time. It would therefore be desirable to develop a less complex fiber-optic sensor coil, i.e., one which depends only on pitch angle and not requiring a specific number of turns or twists, yet having a significant effect comparable to circular birefringence that can overcome or xe2x80x9cswampxe2x80x9d any residual linear birefringence of the coil.
According to one aspect of the invention, a fiber optic sensor coil for a current sensor for increased sensitivity may have at least one winding. The sensing fiber may be wound without torsion, i.e., without torsion twists, around a current-carrying wire to form the coil. A helix can be wound with or without torsion, depending on whether the free end of the fiber in the winding process is constrained from rotation, or permitted to freely rotate, respectively. The pitch of a torsionless fiber may be selected to result in a specific phase shift of circularly polarized light propagating through the fiber, wherein the phase shift can be caused by Berry""s phase.
According to another aspect of the invention, a fiber optic interferometric current sensor may include a sensing fiber encircling a current-carrying conductor and wound with a pitch. Two circularly polarized light waves may propagate along the sensing fiber. The pitch of the sensing fiber may be selected so as to provide a Berry""s phase shift of the circularly polarized light waves that may be substantially greater than a phase shift caused by a linear birefringence in the sensing fiber.
Embodiments of the invention may include one or more of the following features. The pitch may be between 20xc2x0 and 70xc2x0, preferably approximately 60xc2x0. A pitch outside this range may be used, though may not provide an optimum effect. The form on which the fiber is wound may be slotted such that the fiber helix may be threaded onto the conductor without breaking the conductor. The sensing fiber may be wound in the form of a bobbin, the fiber having a first section with a first winding direction and a second section with a second winding direction. The pitch of the fiber will necessarily decrease to zero so as to reverse the winding direction. The sense of phase rotation, however, may not be changed. The length of fiber in the reversal region may be minimized, as it does not produce a Berry""s phase shift. The fiber may also be wound in the form of a helix around a torus, which encircles the conductor.
Further features and advantages of the present invention will be apparent from the following description of preferred embodiments and from the claims.